This disclosure relates to a method and system for removing low concentrations of organic contaminants from water streams.
Water often has to be purified before it can be consumed as potable water or used for other purposes requiring removal of contaminants. Organic compounds are a common source of contaminants in water. For example, ground or surface water can be polluted by discharges from industrial processes, accidental spills, or sewer system overflow. Water condensed from closed atmospheres or recycled from aqueous waste streams, such as those found in spacecraft, can contain organic compounds that have to be removed to make the water potable, especially in a closed environment like a spacecraft in which it is desirable to recycle all water.
Organic compounds having relatively large molecular structures (e.g., C4 and larger) are relatively easy to remove from water by conventional methods such as adsorption or ion exchange. These removal methods, however, tend to leave low concentrations of organic compounds, especially low molecular weight compounds, in the water. Low molecular weight organic compounds can be difficult to remove from water streams, particularly those compounds that do not readily ionize. In addition, such compounds are not easy to remove with physical sorbents like activated charcoals, zeolites, or other high surface area sorbents. Oxidative processes can remove aqueous organic contaminants, but many such known processes can have drawbacks such as large physical system footprints, long processing times, high power consumption, requirements for consumables, as well as other performance or reliability issues.
Processes for oxidation treatment of contaminated water, known as wet air oxidation, are described in U.S. Pat. No. 2,665,249 to Zimmermann and U.S. Pat. No. 3,133,016 to Stine et al. The Zimmermann process requires temperatures of at least 450° F. and the Stine process requires temperatures between 200° F. and 600° F. and elevated pressures. Although both processes are capable of reducing the total organic carbon (TOC) content of contaminated water, they are incapable of removing some low molecular weight organic compounds, especially acetic acid. Moreover, the processes are energy intensive and require reactors constructed from materials like titanium to withstand corrosive operating conditions. Supercritical water oxidation is capable of converting aqueous organics, including acetic acid, to their elemental oxides. The process requires temperatures between 750° F. and 1200° F. and pressures between 3200 psig and 4000 psig. These high temperatures and pressures require vessels made with thick walls and costly alloys. As a result, the components of a supercritical water oxidation system are heavy and power consumption is high. Photochemical processes that make use of UV light and a chemical oxidant or a photocatalyst can also convert aqueous organics to their elemental oxides. However, these processes tend to have low conversions and require recycling to obtain complete oxidation of the organic contaminants. Additionally, photocatalysts such as TiO2 are subject to deactivation caused by oxidation of airborne silicone contaminants that oxidize to form silicon-containing deposits on the photocatalyst. U.S. Pat. No. 5,234,584 to Birbara et al. discloses a process and catalyst for oxidation treatment of water for removal of organic contaminants. However, Birbara also requires temperatures of at least 250° F., which not only requires energy for heating the water to such temperatures, but also requires energy and equipment to pressurize the water to at least 65 psia in order to maintain the water below its boiling point.